Method and a converter topology for ensuring charge and discharge through a coil so as to allow simultaneous and independent charge and discharge thereof

ABSTRACT

A method and converter topology for ensuring charge and discharge of electric current to a coil so as to allow simultaneous and independent charge and discharge thereof, particularly suitable for a superconducting coil and showing an increase in power transfer by a factor of up to two as compared with prior art converters.

This application claims the benefit of U.S. provisional application No.60/071,852, filed Jan. 20, 1998.

FIELD OF THE INVENTION

This invention relates to electrical circuits of converters operatingwith the participation of charge and discharge of the electric currentthrough a coil.

BACKGROUND OF THE INVENTION

Many power electric circuits require the conversion of electric powerfrom one form to another. For example, various types of convertercircuits are used as energy stabilizing devices for an AC or DC load.Such converter circuits are typically based on the storage of electricalenergy in a coil during one certain moment and which is transferred to aload during a moment after. The energy is stored as a magnetic fieldaround the coil so that the actual energy transfer realized by thedischarge of the coil comprises the conversion of magnetic energy toelectrical energy.

In most converters, the build-up of magnetic energy through the coil andits subsequent discharge are two independent phases which are performedconcurrently. That is to say, energy is first built-up around the coilas a magnetic field and, during a subsequent independent stage, the coilis connected to an external load so that the stored energy can dischargethrough the load, thereby supplying energy thereto.

U.S. Pat. No. 4,695.932 (Higashino) describes a circuit for storingenergy delivered from an AC supply which comprises a DC capacitor andreversible chopper between an AC/DC reversible conversion circuitconnected to the AC supply and a current supply circuit including asuperconductive coil. This capacitor is connected intermittently to thesuperconductive coil in response to the action of the reversibleconverter, so that when connected it delivers energy to thesuperconductive coil or receives energy therefrom. The reversiblechopper circuit controls in accordance with required values themagnitude of transfer of energy between the DC capacitor and thesuperconductive coil.

By such means, a DC capacitor is intermittently connected to thesuperconductive coil by the action of the reversible chopper circuitwhich controls the coil current flow, and thereby acts so that energy isdelivered to, and released from, the superconductive coil. By suchmeans, energy can be stored in a superconductive coil when demand is lowand can be withdrawn from the coil when demand is high thereby reducinglarge fluctuations in demand and achieving a more stable output.

However, the charge and discharge pulses are applied to the coilconcurrently and this means that the energy stored within the coilcannot be increased during discharge. Since the coil functions as anenergy reservoir, this is not an efficient manner of the energy control.This is somewhat analogous to allowing a water tank to supply water onlywhen there is no income of water and vice versa when obviously it wouldbe preferable to allow charge and discharge to be effectedsimultaneously and independently.

However, no such implementation has been suggested in the prior art.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodof charge and discharge of electromagnetic energy through a coil so asto allow simultaneous and independent charge and discharge thereof.

It is a further object of the present invention to provide a convertertopology for enabling charge and discharge through a coil so as to allowsimultaneous and independent charge and discharge thereof.

According to a broad aspect of the invention there is provided a methodof charge and discharge of electric current through a supper conductingcoil so as to allow simultaneous and independent charge and dischargethereof, the method comprising the steps of:

(a) connecting a power source to the coil via a first switchable path,

(b) connecting a load to the coil via a second switchable path, and

(c) selectively switching said first and second switchable pathaccording to whether it is required to charge or discharge the coilindependently or to charge and discharge the coil simultaneously.

According to a further aspect of the invention there is provided aconverter topology for ensuring charge and discharge of electric currentthrough a supper conducting coil so as to allow simultaneous andindependent charge and discharge thereof, the converter topologycomprising:

at least two switchable paths coupled to the coil and being respectivelyconnected to a power source and a load,

control circuit coupled to said switchable paths for selectivelyswitching thereof according to whether it is required to charge ordischarge the coil independently or to charge and discharge the coilsimultaneously.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how the invention may becarried out in practice, some preferred embodiments will now bedescribed, by way of non-limiting example only, with reference to theaccompanying drawings, in which:

FIGS. 1a and 1b show two typical circuit configurations of a prior artconverter;

FIG. 2 shows schematically a circuit arrangement according to a firstembodiment of the invention;

FIG. 3 depicts the equivalent switching configuration of FIG. 2 duringsimultaneous charge and discharge of the coil;

FIG. 4 shows schematically a system including a converter with asuperconducting coil according to FIG. 2;

FIG. 5 shows schematically a controller for the switching circuit ofFIG. 4;

FIG. 6 is a state table relating to operation of the controller shown inFIG. 5; and

FIG. 7 shows schematically an AC--AC converter arrangement wherein thepower source is 3 phase AC-grid having a 3-phase load directly connectedthereto.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIGS. 1a and 1b show two kinds of prior art converter arrangements. Thefirst shown in FIG. 1a and depicted generally as 10 includes a coil 11having a first end 12 connected to GND and having a second end 13connected via a switch 14 to a Load across which there is connected acapacitor 15. The coil 11 is charged by a power source 16 connectedbetween GND and the second end 13 of the coil via switch 17. The coil 11is charged by closing the switch 17 and opening the switch 14, whereuponthe load L is disconnected from the coil 11 and continues to receiveenergy from the capacitor 15, which is charged by the coil 11 during thetime that the switch 14 is closed and switch 17 is opened.

Therefore, in order to ensure that the load L is not subject tointermittent interruption of energy, the switch 14 must be closed beforethe capacitor 15 becomes discharged below a permitted level. At the sametime, the switch 14 must be left open for a sufficient period of time toallow for the coil 11 to store sufficient charge from the power source16.

In such an arrangement, the coil is therefore either charged (when theswitch 14 is open and the switch 17 is closed) or is discharged (whenthe switch 14 is closed and the switch 17 is open) and the continuityand stability of the energy applied to the load L is dependent only onthe smoothing effect of the capacitor 15.

The second prior art converter arrangement shown in FIG. 1b and depictedgenerally as 110 includes a coil 111 having a first end 112 connected topower source 116 and having a second end 113 connected via a switch 114to a load across which there is connected a capacitor 115. The coil 111is charged continuously by a power source 116 connected between GND andthe end 112 of the coil. The coil 111 is charged only when the switch117 is closed and the switch 114 is open, whereupon the load L isdisconnected from the coil 111 and continues to receive energy from thecapacitor 115 which is charged by the coil 111 during the time that theswitch 114 is closed. In this arrangement independent discharge of thecoil 111 is prohibited.

FIG. 2 shows schematically a converter configuration according to theinvention depicted generally as 20 and including a coil 21 having afirst end 22 which is connected to a power source 23 via a switch S₄, asecond end of the power source 23 being connected to GND. A second end24 of the coil 21 is connected, via a switch S₂ to a load L which, inturn, is connected to GND and across which is a smoothing capacitor 25.

The first end 22 of the coil 21 is connected via a switch S₃ to GND,whilst the second end 24 of the coil 21 is connected via a switch S₁ toGND.

Such a circuit configuration allows for four different states dependingon whether the switches S₁ to S₄ are opened or closed. Thus, if it isdesired to charge the coil 21 whilst preventing discharge thereofthrough the load L then, as shown in FIG. 6, switches S₁ and S₄ must beclosed whilst switches S₂ and S₃ are opened. On the other hand, when itis desired to discharge the coil 21 through the load L whilst preventingcharge of the coil 21, then switches S₁ and S₄ are opened whilstswitches S₂ and S₃ are closed.

The configuration shown in FIG. 2 also allows for the coil 21 to becharged by the power source 23 whilst, at the same time, dischargingthrough the load L. In this situation, switches S₁ and S₃ are openedwhilst switches S₂ and S₄ are closed.

Preferably, this coil 21 is a superconducting coil with effectively zeroenergy losses. In this case opening switches S₂ and S₄ , the load L andthe power source are disconnected from the coil 21 whilst switches S₁and S₃, being closed, shorts the two ends 22 and 24 of the coil therebyallowing electric current to flow losslessly through the coil so as tomake energy available for discharging to the capacitor 25 and feedingthe load L when the switch S₁ is eventually opened and the coil 21connected across the load L.

FIG. 3 shows the effect in FIG. 2 of closing switches S₂ and S₄, whilstswitches S₁ and S₃ are opened, whereby it can readily be seen that theload L receives charge from the coil 21 which is connected to GND viathe power source 23. Consequently, in such a configuration, the coil 21both charges and discharges simultaneously.

FIG. 4 shows a more detailed diagram of a switching circuit havingswitches S₁, S₂, S₃ and S₄ as depicted in FIG. 2 but further showingdetails, albeit schematically, of a protection circuit 30 and acontroller 31.

FIG. 5 shows schematically a functional representation of the controller31 employing four switch drivers 35 36, 37 and 38 which are responsivelycoupled to a TIME RESTRICTIONS & GLUE LOGIC CIRCUIT 40 for controllingthe four switches S₁ to S₄ shown in FIG. 4.

The TIME RESTRICTIONS & GLUE LOGIC CIRCUIT 40 is responsive to a modeselection 41 as well as to three inputs OV, OF and OC representative ofthe overvoltage, overfield and overcurrent across or through the coil 21respectively, for switching the respective switch drivers 35 to 38 inorder that the desired configuration of the switching circuit will beachieved.

The voltage on the load and field of the coil 21 are derived byrespective voltage and field sensors 42 and 43 whilst the currentflowing through the coil 21 is derived by a current sensor 44. The fieldand current sensors 43 and 44 are connected to respective amplifiers 45and 46. The voltage signal derived by the voltage sensor 42 as well asthe amplified field and current signals derived at the output of therespective amplifiers 45 and 46 are fed to the non-inverting inputs ofrespective comparators 47, 48 and 49.

Respective inverting inputs of the three comparators 47, 48 and 49 areconnected to a voltage, field and current setting device 50 for settingthe desired voltage, field and current thresholds. Thus, the threedesired set signals OV, OF and OC are derived at the respective outputsof the comparators 47, 48 and 49 and are fed to the time restrictions &glue logic circuit 40 for controlling the four switch drivers 35 to 38.

The control circuit thus provides control signals for the four switchesS₁ to S₄ in order to achieve the desired characteristics of the inputcurrent and to yield desired output voltages and energy level in thecoil 21. As a result of this optimization, the power transferred by thecoil 21 is increased by a factor of up to 2 with respect to prior artdevices.

FIG. 6 summarizes the status of the four switches S₁ to S₄ according towhether the coil 21 is to be operated in charge only mode, dischargeonly mode, charge and discharge mode or persistence mode, these modescan also be selected manually by the mode selector switch 41 shown inFIG. 5.

However it is also contemplated that some of the switches S₁ to S₄ maybe replaced by rectifier diodes which allow current to flow in only onedirection and thus may behave as open or closed switches depending onthe voltage polarity.

FIG. 7 shows the topology of the AC--AC converter depicted generally as70 containing a set of eight switches S₁ to S₈ directly connected to a3-phase AC-grid having an incoming 3-phase feeder 71. A line filter 72is connected across the incoming 3-phase feeder 71 and a load 73 isconnected in parallel with the switches S₁ to S₈. A coil 74 has one endconnected to the four switches S₁ to S₄ and has its other end connectedto the four switches S₅ to S₈. A switch S₉ ensures `persistent current`mode of the operation of the coil 74.

At any moment according to the situation of a particular phase the coil74 may be charged or discharged from this phase by connecting theappropriate end of the coil to this phase and the second end to the starpoint "0" thereby enabling charge or discharge of the coil 74. Accordingto the situation of a second phase at the same moment, the second end ofthe coil 74 can be connected to this second phase for simultaneouscharge and discharge of the coil or charge/discharge from both phases.

In addition to the configurations described above, a DC power source canalso be employed together with either a single-phase or 3-phase AC line.In either case any combination of DC load, single-phase AC load or3-phase AC load may be connected thereto.

We claim:
 1. A method of charge and discharge of electric currentthrough a superconducting coil so as to allow simultaneous andindependent charge and discharge thereof, the method comprising thesteps of:(a) connecting a power source to the coil via a firstswitchable path, (b) connecting a load to the coil via a secondswitchable path, and (c) selectively switching said first and secondswitchable path according to whether it is required to charge ordischarge the coil independently or to charge and discharge the coilsimultaneously.
 2. A converter topology for ensuring charge anddischarge of electric current through a superconducting coil so as toallow simultaneous and independent charge and discharge thereof, theconverter topology comprising:at least two switchable paths coupled tothe coil and being respectively connected to a power source and a load,control circuit coupled to said switchable paths for selectivelyswitching thereof according to whether it is required to charge ordischarge the coil independently or to charge and discharge the coilsimultaneously.
 3. The converter topology according to claim 2, whereinat least one of the switchable paths contains a rectifier diode which isbiased to block current flow of a predetermined polarity.
 4. Theconverter topology according to claim 2, further including a thirdswitchable path connected across the coil and wherein the controlcircuit is adapted to close the third switchable path so as to shortsaid coil thereby producing "persistent current" through the coil. 5.The converter topology according to claim 4 for use with an AC powerline.
 6. The converter topology according to claim 4 for use with a DCpower line.
 7. The converter topology according to claim 4, wherein thepower source is a single phase AC-grid.
 8. The converter topologyaccording to claim 4, wherein the power source is a three phase AC-grid.9. The converter topology according to claim 8, wherein the load is a3-phase load directly connected to a 3-phase AC-grid.
 10. The convertertopology according to claim 2, where the switchable paths include:afirst switchable path independently and selectively connecting oppositeends of the coil to the power source and GND, and a second switchablepath independent of the first switchable path, independently andselectively switching opposite ends of the coil to respective terminalsof the load.
 11. The converter topology according to claim 10, whereinat least one of the switchable paths contains a rectifier diode which isbiased to block current flow of a predetermined polarity.
 12. Theconverter topology according to claim 10, further including a thirdswitchable path connected across the coil and wherein the controlcircuit is adapted to close the third switchable path so as to shortsaid coil thereby producing "persistent current" through the coil. 13.The converter topology according to claim 12, for use with an AC powerline.
 14. The converter topology according to claim 12, for use with anDC power line.
 15. The converter topology according to claim 12, whereinthe power source is a single phase AC-grid.
 16. The converter topologyaccording to claim 12, wherein the power source is a three phaseAC-grid.
 17. The converter topology according to claim 16, wherein theload is a 3-phase load connected to a 3-phase AC-grid.